Dynamical Simulations of Magnetically Channeled Line-Driven Stellar Winds: I. Isothermal, Nonrotating, Radially Driven Flow

Abstract

We present numerical magnetohydrodynamic (MHD) simulations of the effect of stellar dipole magnetic fields on line-driven wind outflows from hot, luminous stars. Unlike previous fixed-field analyses, the simulations here take full account of the dynamical competition between field and flow, and thus apply to a full range of magnetic field strength, and within both closed and open magnetic topologies. A key result is that the overall degree to which the wind is influenced by the field depends largely on a single, dimensionless, `wind magnetic confinement parameter', $η_{\ast}$ ($ = B_{eq}^2 R_\ast^2/{\dot M} v_\infty$), which characterizes the ratio between magnetic field energy density and kinetic energy density of the wind. For weak confinement $η_{\ast} \le 1$, the field is fully opened by the wind outflow, but nonetheless for confinements as small as $η_{\ast}=1/10$ can have a significant back-influence in enhancing the density and reducing the flow speed near the magnetic equator. For stronger confinement $η_{\ast} > 1$, the magnetic field remains closed over a limited range of latitude and height about the equatorial surface, but eventually is opened into a nearly radial configuration at large radii.